Dutch Elm Disease

Dutch Elm

Dutch Elm Disease (DED) is a fatal, debilitating fungal disease that affects elm species, with particular mortality for the American Elm (Ulmus americana) and European Elm (Ulmus glabra). DED is caused by the fungal pathogen Ophiostoma ulmi (or Ophiostoma novo-ulmi in its variant form) through elm bark beetles, which act as the vector for the fungus; on the eastern side of North America, it is the native elm bark beetle that acts as the most important and often primary vector. Across eastern Ontario, and indeed most of Canada, Dutch Elm Disease has ravaged elm trees, particularly in urban and suburban environments.

Beginning Signs :

Wilting and Yellowing of Leaves: The disease presents with withering and yellowing leaves of one or several limbs that usually begin at the tops of the tree. The leaves will then turn brown and shrivel as they dry.
Uneven Symptoms: Unlike some other diseases, DED often has an uneven impact on the limbs, so that you often will not see symptoms on all parts of the tree at the same time. It is common for DED to only start with wilting on one side or one of the limbs.
Browning of xylem: If you cut the limb, you can start to see the dark streaks through the wood, but if you look closely, you will see these streaks through the cortex. The dark streaks represent the fungus obstructions of the xylem, starvation the tree of water and nutrients.
Defoliation: The defoliation process usually becomes evident in the latter part of the summer, and the affected branches will have lost their foliage early. You should begin to see sparser appearances of the tree.

Advanced Symptoms :

Rapid Decline: If left untreated, this disease can rapidly advance to the death of large parts or the entire tree. Within a few months to a year, the tree can die because the fungus can completely block the movement of water and nutrients through the vascular system of the tree.
Dieback: The fungus often moves quickly into the vascular system from the branches into the main trunk. In the advanced parts of the disease, you may see the entire tree die due to the vascular system being compromised and allowing rapid dieback.
Bark and Beetles: You will sometimes see small holes in the trees from where elm bark beetles have been feeding on the tree bark. The beetles carry the fungus from tree to tree.

How Dutch Elm Disease Spreads

Elm Bark Beetles: The main way they spread the disease is with beetles, especially Hylurgopinus rufipes and Scolytus species of beetles. These beetles become infected when feeding on infected trees’ sap. When the beetles feed on a healthy tree, they move the fungal spores onto the healthy trees through the bark.
Root Grafting: Besides beetles being the main vector of spread, the disease can spread from nearby trees through grafted roots, especially in areas with many elms.

Disposal of Infected Trees :

Immediate Disposal: If you suspect that a tree has Dutch Elm Disease, you want to remove the tree as soon as you can. Cut the tree down and dispose of all of the infected wood (branches, logs, etc). Your best disposal option is probably going to be burning it or chipping it up to destroy the wood and just help prevent the spread of the disease.
Disinfecting Tools: When you are pruning the infected wood, it is best to disinfect your tools after each cut. This will avoid transferring the fungus to healthy trees.

Fungicide Treatments:

Injection: If you catch the infection early, there are systemic fungicides that can be injected into the tree to help retain healthy portions of the tree and slow the onset of the disease. Fungicides can also be effective if applied in a preventive method or in an early stage of the disease.
Spraying: In urban scenarios where there is some awareness about previous outbreaks, some municipalities have treated trees with fungicide on an ongoing preventive basis, especially for high-value trees on public green spaces.

Management of the elm bark beetle:

Insecticide treatment: Since controlling the insect is necessary to stop the spread of Dutch elm disease, the use of insecticides on trees can potentially kill beetles or reduce the number of beetles.
Trap trees: You can use trap trees by pruning the affected branches in late spring to early summer, and then the “trap trees” can be cut down and disposed of once the beetles have become attracted to the affected wood.

Pruning:

Prune Correctly: Prune the infected branches early in the year, and always in winter, in order not to attract beetles. Make sure to disinfect the pruning tools between cuts to avoid cross-contamination.

How to Prevent Dutch Elm Disease

Preventive Fungicide Treatments:

Fungicide Injection: Fungicide injections can be performed in the event there is a disease outbreak on elm trees in your area. Injectable principles can help protect trees from disease in reasonably high-risk areas between every two and three years.

Controlling Elm Bark Beetles:

Insecticides: If an elm bark beetle issue is identified in your area, applying insecticides during beetle season (generally spring and early summer) may reduce the beetle population. If the trees are infected, insecticide should be applied to both infected trees and the healthy trees nearby to help prevent the spread of the disease.
Sanitation: To minimize the chance of attracting beetles, prune off and dispose of dead wood and/or dying live branches in late winter or in early spring. Be sure infected tree wood is disposed of in such a way that it cannot be used as a breeding material for elm bark beetles.

Tree Health Management:

Proper Tree Care: Using proper tree care practices such as watering, mulching, and fertilizing trees will greatly improve the overall health of the tree and increase the tree’s ability to repel disease and recover from infection or stress. Stressed trees (from drought, mechanical injury, etc.) will have a higher potential to contract Dutch Elm Disease.
Don’t Prune During Active Beetle Season: If you must prune elms, you should prune elms in the dormant season (winter) when the beetle is not active. You should never prune elms during the active season (spring and summer).

Pruning:

Prune Properly: If any of the branches are infected, you should prune the branch as soon as possible, but always in the winter to eliminate the possibility of an attraction for the beetles. Be sure to clean and disinfect all pruning tools between cuts to eliminate the possibilities of cross-contamination.

Monitoring and Early Discovery:

Regular Inspections: Elm trees should be regularly examined for signs of Dutch Elm Disease, especially wilting or yellowing leaves, and/or any changes in the color of the bark. The earlier the disease is detected, the better to limit its spread.

If you suspect an infection or need help determining what to look for, a certified arborist can take a look at your trees. Once a professional visits, he or she will be able to help with diagnosis and take samples for laboratory testing, provide treatment options, and offer advice for maintaining a healthy tree.

Conclusion

Dutch Elm Disease is a lethal fungal infection that can theoretically decimate entire populations of elm trees without treatment. There are several components of management of this disease, including prevention, early detection, and treatment. If elms have been infected and infested, the spread of Dutch Elm Disease can be managed to some extent by controlling elm bark beetles, maintaining tree health, and applying fungicide treatments. If you recognize symptoms of DED on your trees, you will need to act quickly to eliminate the infected trees to stop the spread of the infection, along with protecting other elms.

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